Durring the summer I took the kids swimming on the river Avon. Whilst there we saw some kids throwing some water bombs so after a quick barter and in exchange for some Jaffa cakes I had some and more importantly the source.
They cost £1, come in a pack of 100 with filling nozzle that screws onto a standard lemonade bottle, varriety of colours and will fill up to about grapfriut size. Best size is about the size of a roes heart. Readily available from most corner shops and ASDA stock them in the summer.
Simply fill with water and hang with a bit of string and off you go. When filled with water they hang well and dont blow about. They are excellent for the kids with the .22lr as explode when hit.
In the summer I was having a bit of practice out at 300yds, so placed a large piece of cardboard in front of a backstop then hung some of the ballons in front of both. Took a bit to get the elevation correct but you clearly see the strike at 300, saves a lot of walking. Best thing cheap as chips and very portable.